1. Field of the Invention
The invention relates to a display device comprising a display tube having at least one control electrode for generating an electron beam, a display screen for displaying a video signal and an inner coating at the inside of the display tube and an outer coating at the outside of the display tube, a deflection unit for deflecting the electron beam across the display screen, said deflection unit comprising at least a line deflection coil, a field deflection coil and a line output transformer having a primary winding coupled to a line deflection section of the deflection unit and a secondary winding having a first connector coupled to a ground terminal and a second connector coupled to the inner coating of the display tube for generating a high DC voltage.
2. Description of the Related Art
Such a display device is generally known. For example, U.S. Pat. No. 4,315,306 describes a line output transformer (LOT) for use in a display device as described above. The secondary winding of the line output transformator generates the high DC voltage (EHT) which is required for display. The display tube may be the picture tube of a television receiver or monitor. For this type of equipment, there are safety regulations specifying that certain electrical fields in the vicinity of the device may not exceed certain limits because such fields can be hazardous to the health of people using these devices. For example, the AEF2, which is the term used for the alternating electrical field in band 2 (the frequency band comprising the horizontal deflection frequencies), may not exceed 2.5 V/m at a distance of 0.5 m from the center of the screen of the display tube.
In general, all parts having an alternating voltage with a frequency in band 2 will contribute to the AEF2. The back and the sides of a monitor/television receiver can easily be shielded by means of a metal housing or an electrically conductive coating. It is more difficult to shield the front side of the display tube because this side comprises a glass display screen. The inner side of the cone and the inner side of the display screen of the display tube are electrically conductive and are connected to the anode contact to which the high voltage conductor of the line output transformer is to be connected. Ripple on the high DC voltage EHT is thus coupled to the inside of the display screen and consequently the display screen acts as a generator for the AEF2 at the front side of the display tube. It has been found that a ripple of 15-25 V on the EHT of about 20 kV occurs in current commercially available monitors. Depending on the display screen size, this ripple will cause an AEF2 value which exceeds the safety requirements. For example, a ripple of 21 V and an AEF2 value of 4 V/m have been found on a 17 inch monochrome monitor. The ripple is partly due to the internal stabilization of the EHT, but a major part is caused by capacitive coupling of the flyback pulse on the horizontal deflection coil(s) to the inner coating of the display tube. In principle, it would be possible to reduce the AEF2 value by shielding the display tube and/or the display screen by means of an electrically conductive coating. Although such coatings are available, they are very expensive and not suitable for mass production. Therefore, it would be preferable to eliminate or reduce the cause of the AEF2. This could be achieved by providing an electrically conductive layer between the deflection unit and the display tube. However, it is difficult to obtain a satisfactory isolation between this layer and the horizontal deflection coil(s). Moreover, this conductive layer must have a conductivity which is sufficient to perform its shielding function, but low enough to prevent excessive eddy currents. As it is difficult to meet all these requirements, this solution has not been used in practice.